Bitumen composition

ABSTRACT

A bitumen composition comprising an asymmetric, radial polymer containing a plurality of block copolymer arms comprising at least one polymeric block of a monoalkenyl aromatic hydrocarbon and at least one polymeric block of a conjugated diolefin polymer and a plurality of conjugated diolefin polymer arms. The bitumen composition exhibits increased toughness and tenacity as determined by the Benson method and is particularly useful in road paving applications. Best results are achieved when the asymmetric, radial block copolymer is sulfur vulcanized after incorporation into the bitumen composition.

This is a continuation of application Ser. No. 07/121,426, filed Nov.16, 1987, now abandoned.

BACKGROUND

1. Field of the Invention

This invention relates to a bitumen composition. More particularly thisinvention relates to a composition comprising bitumen and a polymericmodifier.

2. Prior Art

Bitumen compositions, particularly asphalt compositions, comprising oneor more polymeric modifiers are, of course, well known in the prior art.In general, the addition of a polymeric modifier to a bitumencomposition, particularly an asphalt composition, will improve one ormore of the bitumen properties such as penetration, softening point,toughness, tenacity, heat resistance and the like such as taught inJapanese Patent No. Sho 58(1983)-47057. Suitable polymeric additivesinclude: symmetric radical block copolymers comprising arms having dienepolymer blocks and vinyl aromatic polymer blocks such as taught in U.S.Pat. No. 4,217,259; block copolymers of a monoalkenyl aromatichydrocarbon monomer and a conjugated diolefin such as taught in U.S.Pat. No. 4,585,816 and the patents therein cited; olefin homopolymersand copolymers, particularly 1-butene homopolymers and copolymers, astaught in U.S. Pat. No. 3,915,914 and the patents therein cited; andisoolefin homopolymers, particularly polyisobutylene, such as taught inU.S. Pat. No. 3,615,803.

It is, of course, well known in the prior art to use asphaltcompositions comprising polymeric modifiers in the preparation of roadsurfaces. In general, such road surfaces provide satisfactory service,at least on low traffic roads, for extending periods of time. Failure ofsuch services is, however, inevitable, failures generally occur throughbleeding, fatting up, cracking and chip loss. As is well known, the timeuntil failure varies with several factors such as the amount and weightof traffic and various weather factors such as temperature and rainfall.Unfortunately, failure frequently occurs at shorter time intervals thandesired. Such failures are due primarily to the poor toughness andtenacity of the asphalt composition. This is particularly true onheavily traveled roadways. The need, then, for an improved asphaltcomposition having improved toughness and tenacity thereby extending thetime period between failures is believed to be readily apparent.

SUMMARY OF THE INVENTION

It is now been discovered that the foregoing and other disadvantages ofthe prior art bitumen compositions can be avoided or at leastsignificantly reduced with the bitumen composition of the presentinvention and an improved bitumen composition having increased toughnessand tenacity provided thereby. It is, therefore, an object of thisinvention to provide an improved bitumen composition. It is anotherobject of this invention to provide a bitumen composition havingincreased toughness. It is still another object of this invention toprovide a bitumen composition having increased tenacity. It is yetanother object of this invention to provide a bitumen composition thatcan be used in the preparation of road surfaces capable of handlingeither more or heavier traffic. It is even another object of thisinvention to provide a bitumen composition useful in the preparation ofroad surfaces which will have a longer life before significant failure.The foregoing and other objects and advantages will become apparent fromthe description of the invention set forth hereinafter.

In accordance with the present invention, the foregoing and otherobjects and advantages are accomplished with a bitumen compositioncomprising an asymmetric, radial block copolymer comprising a pluralityof arms containing at least one block of a predominantly monoalkenylaromatic hydrocarbon polymer and at least one block of a predominantlyconjugated diolefin polymer and a plurality of arms of a polymercomprising predominantly only conjugated diolefins, said asymmetric,radial block copolymer having a weight average molecular weight of atleast 600,000. The arms containing predominantly only conjugateddiolefins may be a homopolymer of a conjugated diolefin or a copolymerof two or more conjugated diolefins. As used herein, the recitation"predominantly" is intended to mean that the specified monomer will bethe dominant monomeric component (at least 85%) but not necessarily thesole monomeric component. The asymmetric radial block copolymer may becrosslinked after it is incorporated into the bitumen composition.

DETAILED DESCRIPTION OF THE INVENTION

As indicated supra, the present invention is drawn to a bitumencomposition comprising an asymmetric block copolymer comprising aplurality of polymeric arms containing at least one monoalkenyl aromatichydrocarbon polymer block and at least one conjugated diolefin block anda plurality of arms which are, predominantly at least, polymers of oneor more conjugated diolefins, said asymmetric, radial block copolymerhaving a weight average molecular weight of at least 600,000. Ingeneral, the bitumen composition of this invention will be useful in anyof the applications known in the prior art for such compositions, but asindicated more fully hereinafter, maximum advantage is realized when thebitumen composition is used in the paving of road surfaces. As alsoindicated more fully hereinafter, the bitumen composition may compriseother materials such as fillers, aggregate, pigments, other synthetic ornatural resins, stabilizers, fire retardants, and the like.

In general, any of the natural and synthetic bitumens known in the priorart are suitable for use in the composition of the present invention.Suitable bitumens, then, include natural and synthetic asphalts such asnative, rock and lake asphalts as well as petroleum asphalts. Thebitumen used in the composition may be neat, highly cracked, residual orair blown. In general, bitumens useful in the composition of the presentinvention will have penetrations (ASTM method D5) within the range fromabout 10 to about 800 and, softening points within the range from about30° C. to about 115° C. Petroleum residual asphalts are particularlyuseful in the composition of the present invention and are, therefore,preferred.

In general, the asymmetric, radial polymers useful in the bitumencompositions of this invention may be represented by the followinggeneral formula:

    (A--B.sub.x YC).sub.z

wherein:

A--B is a block copolymer arm;

A is a monoalkenyl aromatic hydrocarbon polymer block;

B is a conjugated diolefin polymer block which may be at least partiallyhydrogenated;

C is a conjugated diolefin polymer arm which may contain the same or adifferent conjugated diolefin as is contained in B and which may be atleast partially hydrogenated;

x+z equals a number from about 6 to about 30;

x/z ranges from about 5 to 1 to about 1 to 5; and

y is the residue of a multifunctional coupling agent.

Such asymmetric polymers are within the teaching of U.S. Pat. Nos.4,163,764; 4,391,949, 4,444,953 and Canadian Patent No. 997,889, thedisclosure of which patents are herein incorporated by reference.

Monoalkenyl aromatic hydrocarbon monomers useful in preparing themonoalkenyl aromatic hydrocarbon polymer blocks include styrene, alkylsubstituted styrenes, paramethyoxystyrene, vinyl naphthalene, vinyltoluene and the like. Conjugated diolefins useful in preparing theconjugated diolefin polymer blocks include the conjugated diolefinshaving from 4 to about 12 carbon atoms such as 1,3-butadiene, isoprene,piperylene, methylpentadiene, phenylbutadiene,3,4-dimethyl-1,3-hexadiene, 4,5-diethyl-1,3-octadiene and the like.

In general, suitable asymmetric, radial polymers may be prepared byfirst preparing a suitable mixture of living polymeric arms and thenreacting the mixture with a polyfunctional (multifunctional) couplingagent. As is well known in the prior art, and taught in the justabove-referenced U.S. patents, the living polymer arms may be preparedvia anionic polymerization. The different arms may be preparedseparately and then combined, or, in effect, prepared simultaneously byadding additional catalysts or initiator after preparation of themono-alken aromatic hydrocarbon polymer block has been completed andjust before the conjugated diolefin block is grown in the blockcopolymer. Suitable asymmetric, radial polymers may also be prepared byfirst coupling a plurality of one of the arms and then, in effect,growing the other from the lithium sites incorporated into the nucleusas a result of the initial coupling. This method of preparingasymmetric, radial polymers is taught, for Example, in U.S. Pat. Nos.3,985,830 and 4,108,945, the disclosure of which patents is hereinincorporated by reference.

In general, the asymmetric, radial polymers useful in the bitumencomposition of this invention will comprise from about 2 to about 25block copolymer arms, on average, comprising at least one monoalkenylaromatic hydrocarbon polymer block and at least one conjugated diolefinpolymer block and from about 2 to about 25 arms, on average, which are aconjugated diolefin polymer, the ratio of block copolymer arms toconjugated diolefin polymer arms being within the range from about 5:1to about 1:5. In general, each of the monoalkenyl aromatic hydrocarbonpolymer blocks will have a weight average molecular weight within therange from about 2,000 to about 50,000 and each of the conjugateddiolefin polymeric blocks will have a weight average molecular weightwithin the range from about 5,000 to about 150,000. In general, theconjugated diolefin polymer arms will have a weight average molecularweight within the range from about 5,000 to about 150,000. Theasymmetric, radial polymers useful in the bitumen composition of thisinvention, will, however, have a weight average molecular weight withinthe range from about 600,000 to about 2,500,000. It will, therefore, benecessary to control both the number of arms and the molecular weightsthereof, within the foregoing ranges, so as to insure that the weightaverage molecular weight of the final polymer, before crosslinking, iswithin the range of about 600,000 to about 2,500,000. The molecularweight of the several arms need not, of course, be the same. In fact, auseful asymmetric, radial polymer could not be prepared with all of itsarms having molecular weights at or near the lower portion of theforegoing molecular weight ranges. Similarly, a final polymer havingmore than about 16 arms, each arm having a molecular weight at or nearthe top of the foregoing molecular weight ranges, would not be useful inthe bitumen composition of this invention.

The asymmetric, radial polymer may, but need not, be hydrogenated suchthat up to about 98% of the residual unsaturation contained in both theconjugated diolefin polymer blocks of the copolymer arms and theconjugated diolefin polymer arms is eliminated via saturation thereof.The hydrogenation will be accomplished selectively such that less thanabout 5 wt% of the unsaturation contained in the monoalkenyl aromatichydrocarbon blocks is converted or saturated. In a most preferredembodiment of the invention, the selective hydrogenation will beaccomplished via the selective hydrogenation technique taught in U.S.Pat. No. 3,700,633. When selective hydrogenation is accomplished, careshould be exercised to accomplish the hydrogenation in such a manner asto retain sufficient unsaturation to permit sulfur vulcanization ifsulfur vulcanization is to also be accomplished in a manner ashereinafter taught.

In general, the bitumen composition of this invention will comprise fromabout 1 to about 20 parts, by weight, of asymmetric, radial polymer per100 parts, by weight, of bitumen in said composition. Surprisingly, ithas been discovered that bitumen compositions comprising from about 1 toabout 12 parts, by weight, of asymmetric, radial polymer per 100 parts,by weight, of bitumen exhibit a significant increase in toughness andtenacity, as determined using the Benson method, Instron tester, in-lbat 77° F., 20 in-min pull with a tension head having 2.223 cm diameter,when compared to bitumen compositions comprising the same concentrationof a more conventional, prior art polymeric modifier. The difference intoughness and tenacity values so determined, however, diminishes rapidlyat concentrations of asymmetric, radial polymer above about 12 parts, byweight, per 100 parts, by weight, of bitumen until the values oftoughness, tenacity and other key properties reach values comparablethose of the more conventional, prior art polymeric modifiers. Bitumencompositions comprising from about 1 to about 12 parts, by weight, ofasymmetric, radial polymer per 100 parts, by weight, bitumen, aretherefore, preferred and compositions comprising from about 3 to about 6parts, by weight, per 100 parts, by weight, of bitumen are mostpreferred.

In general, the asymmetric, radial polymer useful in the bitumencompositions of this invention may be combined with the bitumen usingany of the methods known heretofore in the prior art for combining suchmaterials. Suitable methods, then, include simple admixture of thecomponents at ambient conditions, admixture with both components in amolten state with agitation and admixtures of solutions of bothcomponents. As is known in the prior art, when one or both of thecomponents is admixed in the solid state it will be advantageous tofinely divide the solid component or components prior to admixture. Asis also well known in the prior art, molten phase admixture can beaccomplished by heating both components to a temperature above theirrespective melting points. As is further known in the prior art,solution admixture may be accomplished by dissolving the two componentsin a suitable solvent, which solvent may be the same or different, andthereafter evaporating the solvent. As indicated more fully hereinafter,admixture of solid polymer components in the molten asphalt phase isgenerally the most conventional way to combine the components and is,therefore, preferred.

After the asymmetric, radial block copolymer has been incorporated intothe bitumen, the same way, but need not, be sulfur vulcanized using thetechniques known in the prior art, such as the method taught in U.S.Pat. No. 4,145,322, the disclosure of which patent is hereinincorporated by reference, to further improve various properties of thecomposition such as both toughness and tenacity. Best results are,however, realized when the asymmetric, radial polymer is sulfurvulcanized after its incorporation into the bitumen composition. Sulfurvulcanization is, therefore, preferred. In general, sulfur vulcanizationcan be accomplished by incorporating from about 0.5 to about 20 mols ofelemental sulfur, as S₈, per mol of asymmetric radial block copolymerinto the preformed bitumen composition and then heating the compositionto a temperature within the range from about 160° C. to about 180° C.The sulfur vulcanization can be accomplished at essentially anypressure.

As indicated supra, the bitumen composition of this invention maycomprise other additives. The other additives actually incorporated,will, of course, depend upon the ultimate end use of the composition.For example, if the bitumen composition is to be used as a roofingcomposition, suitable fillers, pigments, fire retardants and the likemay be incorporated. As is well known in the prior art, suitable fillersinclude asbestos (both as short and long fibers), magnesium silicate,calcium carbonate, micronized silicas, barium sulfate and varioushydrated clays. As also well known in the prior art, suitable pigmentsinclude carbon black, titanium oxide and the like. As further known inthe prior art, suitable fire retardants include halogenated,particularly chlorinated, elastomers and rubbers. In general, suchadditives will be used at concentrations well known in the prior art. Asanother example, the bitumen composition of this invention may be usedas a coating for various substrates. Again, suitable filters, pigments,fire retardants and the like may be incorporated. As a further example,the bitumen composition of this invention may be used to prepare anasphalt concrete composition which in turn will be used as a pavingmaterial. As indicated supra, the bitumen composition of this inventionis particularly effective when used in a road surface and such use is,therefore, preferred.

In general, asphalt concrete compositions may be prepared simply bycombining the bitumen composition of this invention with a suitableaggregate such as chat, sand, screened pebbles, rock and the like. Ingeneral, the particle size of th®aggregate will vary depending upon theparticular application in which the asphalt concrete is used. For pavingapplications, individual states and localities have their ownspecifications which define the aggregate as a mixture of variousparticle size materials combined such that the void spaces will varyfrom about 3 to about 25%. A 6% void is generally considered normal formost highway and street paving applications. In general, an asphaltconcrete within the scope of this invention will comprise from about 4to about 10 wt% of a bitumen composition within the scope of thisinvention and from about 96 to about 90 wt% aggregate. Such an asphaltcomposition would, then, comprise from about 3.3 to about 9.96 wt%bitumen, from about 0.04 to about 1.7 wt% asymmetric, radial polymer andfrom about 90 to about 96 wt% aggregate. Asphalt concrete compositionsmay be prepared according to any of the techniques well known in theprior art, such as the methods summarized in U.S. Pat. No. 4,217,259,the disclosure of which patent is herein incorporated by reference.

PREFERRED EMBODIMENT OF THE INVENTION

In a preferred embodiment of the present invention, a petroleum residualasphalt having a penetration within the range from about 75 to about 400(ASTM method D5) and, a softening point within the range from about 30°C. to about 75° C. will be combined with from about 1 to about 12 parts,by weight, per 100 parts by weight, of asphalt, most preferably withfrom about 3 to about 6 parts, by weight, per 100 parts by weight ofasphalt of an asymmetric, radial polymer having a plurality of blockcopolymer arms and a plurality of homopolymer arms. In the preferredembodiment, the asymmetric, radial polymer will have from about 8 toabout 20 total arms. In the preferred embodiment, the block copolymerarms will comprise one block of polystyrene and one block ofpolybutadiene or polyisoprene and the homopolymer arms will be ahomopolymer of either butadiene or isoprene. In the preferredembodiment, the ratio of block copolymer arms to homopolymer arms willbe within the range from 3:1 to 1:3. In the preferred embodiment, thepolystyrene blocks will have a molecular weight within the range fromabout 3,000 to about 30,000, most preferably 10,000 to 25,000 and theconjugated diolefin polymeric block will have a molecular weight withinthe range from about 15,000 to about 100,000, most preferably 25,000 to85,000. Also, in the preferred embodiment, the conjugated diolefinhomopolymer arms will have a molecular weight within the range fromabout 15,000 to about 100,000, most preferably 25,000 to 85,000. Stillin a preferred embodiment, the asymmetric, radial polymer will have aweight average molecular weight within the range from about 750,000 toabout 1,800,000, the number and the molecular weight of the several armsagain being controlled so as to insure the production of final polymerhaving a weight average molecular weight within this range. In thepreferred embodiment, the asymmetric, radial polymer will be sulfurvulcanized after its incorporation into the bitumen composition.

In a preferred embodiment of the present invention, the preferredbitumen composition will be used in the preparation of an asphaltconcrete. The asphalt concrete will be prepared by combining from about93 to about 95 wt% of an aggregate with from about 5 to about 7 wt% aperformed bitumen composition comprising from about 1 to about 12 parts,most preferable from about 3 to about 6 parts, of an asymmetric, radialpolymer per 100 parts, by weight, asphalt. The resulting asphaltconcrete will, then, contain from about 93 to about 95 weight percentaggregate, from about 4.5 to about 6.95 wt%, most preferable from about4.7 to about 6.8 wt%, asphalt and from about 0.05 to about 0.75 wt%,most preferably from about 0.15 to about 0.4 wt% asymmetric, radialpolymer. The asphalt concrete will be prepared by combining theaggregate and bitumen composition at a temperature within the range fromabout 150° to about 200° C. in a suitable mixing apparatus.

Having thus broadly described the present invention and a preferred andmost preferred embodiment thereof, it is believed that the same willbecome even more apparent by reference to the following Examples. Itwill be appreciated, however, that the examples are presented solely forpurposes of illustration and should not be construed as limiting theinvention.

EXAMPLE 1

In this example, a series of asphalt compositions were prepared usingdifferent block copolymers, one of which was within the scope of thisinvention, or different block copolymer mixtures. With one exception,the asphalt compositions were prepared with both vulcanized andunvulcanized polymers. In those blends when the polymer was vulcanized,elemental sulfur was added to the asphalt composition after it wasformed and the asphalt composition containing the sulfur was held at atemperature within the range from about 160° C. to about 180° C. for onehour. Certain of the asphalt compositions were prepared usingpolymer/oil blends. In these cases, the amount of the polymer/oil blendactually used was adjusted such that the amount of polymer incorporatedinto each the asphalt compositions remained constant at three percent byweight, of polymer/asphalt blend. All of the asphalt compositions wereprepared with an AC-5 asphalt cement. The first of the asphaltcompositions prepared in this example was prepared with a blockcopolymer comprising a single block of polystyrene having a molecularweight of about 15,000 and a single block of polybutadiene having amolecular weight of about 35,000. After preparation of the asphaltcomposition, two moles of sulfur, as S₈, per mole of polymer was addedto the asphalt composition and the composition held at a temperaturewithin the range from about 160° C. to about 180° C. for one hour. Forconvenience, this asphalt composition is hereinafter identified ascomposition 1. The second and third asphalt compositions were preparedwith a tapered diblock polymer having a weight average molecular weightwithin the range from about 50,000 to about 70,000 and containing asingle block of polybutadiene and a single block of styrene. In thefirst of these two compositions, identified hereinafter as composition2A, the polymer was used neat. In the second of these compositions, twomoles of elemental sulfur, as S₈, per mol of polymer, was added to theasphalt composition after preparation and the composition containing theelemental sulfur was held at a temperature within the range from about160° C. to about 180° C. for one hour. For convenience, this compositionis hereinafter identified as 2B. The fourth and fifth bitumencompositions tested in this example were prepared with a triblockpolymer having terminal blocks of polystyrene, each having a molecularweight of about 18,000 and an intermediate block of polybutadiene havinga weight average molecular weight of about 70,000. As used, the polymerwas combined with an equal amount of oil and in both of the bitumencompositions the oil polymer mixture was added to the asphalt in anamount of 6% by weight of oil/polymer/asphalt blend so as to produce ablend containing 3 wt% polymer. In the first of these two bitumencompositions, the polymer was used neat. In the second of these bitumencompositions, after the polymer was dissolved in the asphalt, two molesof elemental sulfur, as S₈, was added to the blend and the blend held ata temperature within the range from about 160°0 C. to about 180° C. forone hour. For convenience, these two bitumen compositions are identifiedas compositions 3A and 3B, respectively. The sixth, seventh and eighthbitumen compositions tested in this example were prepared with a blendof polymers comprising a diblock identical to that used in bitumencomposition 1 and a triblock identical to that used in bitumencompositions 3A and 3B. The polymeric blend, as used, contained an equalweight of an oil. In the first of these three bitumen compositions, thepolymer mixture was used neat. In the second of these compositions, twomoles of sulfur, as S₈, per mole of polymer was added to the bitumencomposition and in the third composition, five moles of sulfur, as S₈,per mole of polymer was added. Both of the compositions to which sulfurwas added were held at a temperature within the range from about 160° C.to about 180° C. for one hour. For convenience, these blends areidentified hereinafter as 4A, 4B and 4C, respectively. The ninth andtenth bitumen compositions tested in this example, were prepared with anasymmetric, radial polymer within the scope of the present invention. Onaverage, the asymmetric, radial polymer used contained 12 blockcopolymer arms each containing a single polystyrene block having amolecular weight of 11,000 and a polyisoprene block having a weightaverage molecular weight of 60,000 and 6 arms which were homopolymers ofisoprene having a weight average molecular weight of 70,000. The averagenumber of total arms (18) was determined using GPC-LALLS. The ratio, onaverage, of twelve copolymer arms to six homopolymer arms was determinedby assuming that this ratio would be the same as it was in the blend ofpolymer arms prior to coupling with divinylbenzene. In the first of thebitumen composition prepared in accordance with this invention, theasymmetric, radial polymer was used neat. In the second of these bitumencompositions, six moles of elemental sulfur, as S₈, per mole of polymerwas added to the bitumen composition after preparation and thecomposition containing the sulfur was held at a temperature within therange from about 160° C. to about 180° C. for one hour. For convenience,these bitumen compositions are hereinafter referred to as compositions5A and 5B, respectively. The eleventh and twelfth bitumen compositionstested in this example were prepared with a diblock polymer identical tothat used in bitumen composition 1. In both of these compositions, thediblock polymer was used as a blend containing 20 wt% naphthenic oil. Inthe eleventh and twelfth bitumen compositions, the polymer oil blend wasadded to the asphalt in an amount of 3.75 wt% based on polymer-oilasphalt blend so as to produce an asphalt composition containing 3 wt%polymer in the blend. In the first of three compositions, the polymerwas used neat. In the second of these compositions, after the bitumencomposition was prepared, two moles of elemental sulfur, as S₈, per moleof polymer was added to the composition and the resulting mixture heldat a temperature within the range from about 160° C. to about 180° C.for one hour. For convenience, these bitumen compositions are identifiedhereinafter as compositions 6A and 6B, respectively. The 13th bitumencomposition tested in this example was identical to the eleventh andtwelfth compositions (6A and 6B) except that an aromatic oil was used inplace of the naphthenic oil. For convenience, the 13th composition isidentified as composition 7. The fourteenth and fifteenth bitumencompositions tested in this Example were prepared with an asymmetric,radial polymer having two styrene-butadiene block copolymer arms and twobutadiene homopolymer arms. The molecular weight of the styrene blockswas about 20,000 while the molecular weight of the butadiene blocks andthe butadiene homopolymer arms was about 36,000. The molecular weight ofthis asymmetric radial polymer was, then, well below that required forthe asymmetric, radial polymer useful in the bitumen compositions ofthis invention. In the first of these bitumen compositions, the polymerwas used neat. In the second of these bitumen compositions, the polymerwas sulfur vulcanized after it was incorporated into the bitumencomposition. For convenience, the fourteenth and fifteenth bitumencompositions are hereinafter referred to as compositions 8A and 8Brespectively. The sixteenth and seventeenth bitumen composition testedin this Example were prepared with asymmetric, radial polymer havingfour styrene-butadiene block copolymer arms. The molecular weight of thestyrene blocks was about 20,000 and the molecular weight of thebutadiene blocks was about 40,000. In the first of these compositions,the polymer was used neat and in the second of these compositions thepolymer was sulfur vulcanized. For convenience, these compositions arehereinafter respectively referred to as compositions 9A and 9B. Theeighteenth and nineteenth bitumen compositions tested in this Examplewere prepared with a symmetric, radial polymer identical to that used incompositions 9A and 9B except that the molecular weight of the styreneblocks was 12,000 rather than 20,000. Again, the polymer was used neatin the first of these compositions and was sulfur vulcanized in thesecond of these compositions. For convenience, these compositions arehereinafter referred to, respectively, as compositions 10A and 10B. Thetwentieth and twenty-first compositions tested in this Example wereprepared with a styrene-isoprene-styrene linear triblock copolymer. Thestyrene blocks each had molecular weights of about 10,000 and themolecular weight of the isoprene block was about 100,000. In the firstof these compositions, the polymer was used neat and in the second ofthese compositions the polymer was sulfur vulcanized after incorporationinto the bitumen composition. For convenience, these compositions arehereinafter, respectively, referred to as compositions 11A and 11B. Allof the asphalt compositions tested in this example were prepared bymixing in a Silverson LDD laboratory mixer equipped with a 1" tubulargeneral purpose disintegrating head turning at 3,000 rpm. Each of theasphalt compositions was prepared at 160° C. and the time required todissolve the polymer in the asphalt was determined. After preparation,each of the asphalt compositions were then tested to determine itspenetration at 25° C. using ASTM Method D5, to determine its ring andball softening point in °C. using ASTM D-36, and to determine itstoughness and tenacity using the Benson method an Instron Tester, in-lbat 77° F., 20 in/min pull. The tension head used in these tests were2.223 cm diameter. The type or mode of failure was also determined. Theresults obtained with each of the compositions as well as otherinformation is summarized in the table below. For comparison purposes,the asphalt used in preparing all of the asphalt compositions wasanalyzed, without any additives, and the values obtained for the asphaltalone are summarized as asphalt composition 12.

                                      TABLE 1                                     __________________________________________________________________________                                                        Were                      Asphalt  Time to  Penetration                                                                           Ring and Ball                                                                           Toughness                                                                           Tenacity                                                                           #    Arms  Failure             Composition No.                                                                        Dissolution, min.                                                                      at 25° C., dmm                                                                 Softening Point, °C.                                                             Kg-cm Kg-cm                                                                              Arms Dissimilar                                                                          Mode                __________________________________________________________________________    1        20-25    106     54        89    53   1    --    Cohesive            2A       60-65    114     51        57    16   1    --    Cohesive            2B       60-65    108     52        73    39   1    --    Cohesive            3A        5-10    112     51        73    48   2    No    Cohesive            3B        5-10    113     54        143   112  2    No    Cohesive            4A        5-10    115     50        99    68   1.5  No    Cohesive            4B        5-10    101     66        148   116  1.5  No    Cohesive            4C        5-10     77     61        78    46   1.5  No    Cohesive            5A       10-15     91     57        150   124  18   Yes   Cohesive            5B       10-15    116     57        237   204  18   Yes   Cohesive            6A       20-25    120     51        58    27   1                              6B       20-25    123     55        81    47   1    --    Cohesive            7         5-10    101     51        69    43   1    --    Cohesive            8A       20-25    109     --        74    36   4    Yes   Cohesive            8B       20-25     90     --        79    47   4    Yes   Cohesive            9A       100-130   91     --        183   144  4    No    Adhesive            9B       100-130   87     --        128   96   4    No    Adhesive            10A      60-65    --      --        70    38   4    No    Cohesive            10B      60-65    --      --        104   70   4    No    Adhesive            11A      20-25    --      --        124   87   2    No    Cohesive            11B      20-25    --      --        110   69   2    No    Cohesive            12                127     47        30     6              Cohesive            __________________________________________________________________________

As will be apparent from the data summarized in the preceding table,inclusion of sulfur into the asphalt composition generally improves boththe toughness and tenacity of the resulting asphalt composition (B v.A), except with compositions 9A and 9B and 11 A and 11B. Too eachsulfur, however, is apparently detrimental (cf. 4C v. 4B). As will alsobe apparent from the data summarized in the preceding table, thecompositions within the scope of the present invention wherein theasymmetric, radial polymer was used neat gave toughness and tenacityvalues equal to or better than the values obtained for thosecompositions containing vulcanized di- and triblock polymers (cf. 5A v.1, 2B, 3B, 4B, 6B, 8B, 9B, 10B and 11B). Moreover, vulcanization of theasymmetric, radial polymer significantly improved both the toughness andtenacity of the asphalt composition (cf. 5B). As will also be apparentfrom the data summarized in the preceding Table, the lower molecularweight asymmetric radial polymer used in Compositions 9 A and 9B gavegood results when used neat but unsatisfactory result after sulfurvulcanization.

EXAMPLE 2

In this Example, four bitumen compositions were prepared wherein apolymeric modifier was incorporated at a concentration of 12 wt% basedon the polymer/asphalt blend. The first and second compositions,hereinafter referred to, respectively, as Compositions 13A and 13B, wereprepared with an asymmetric, radial polymer within the scope of thisinvention and identical to the polymer used in Compositions 5A and 5B ofExample 1. In the first of these compositions, the polymer was used neat(without sulfur curing) while in the second of these compositions, thepolymer was sulfur vulcanized in the same manner as was used in Example1 by incorporating 2 moles of elemental sulfur, as S₈, per mole ofpolymer into the composition. The third and fourth compositions,hereinafter referred to as Compositions 14A and 14B were prepared, witha block copolymer identical to that used in Composition 1 of Example 1.In the first of these compositions, the polymer was used neat while inthe second of these compositions, the polymer was sulfur vulcanized inthe same manner as was used in vulcanizing the polymers in certain ofthe compositions of Example 1 by adding 2 mols of sulfur, as S₈, permole of polymer to the composition. Each of the compositions wereprepared in the same manner as was used in preparation of thecompositions of Example 1. The asphalt used in each composition wasidentical to that used in the compositions of Example 1. Afterpreparation, each of the asphalt compositions were tested to determinethe penetration at 25° C. and the ring and ball softening point in 0 C.using the same test procedures as were used in Example 1. Eachcomposition was also tested to determine its tensile strength in psiusing ASTM D412. The results obtained with each composition aresummarized in the following Table:

                  TABLE                                                           ______________________________________                                                                           Tensile                                    Composition                                                                            Penetration Ring and Ball Strength,                                  No.      at 25° C., dmm                                                                     Softening Point, °C.                                                                 psi                                        ______________________________________                                        13A      58          178           22                                         13B      71          182           38                                         14A      111         191           23                                         14B      74          197           47                                         ______________________________________                                    

As will be apparent from the data summarized in the foregoing Table, thetensile strength of the compositions containing unvulcanized (neat)polymer as well as the tensile strength of the compositions containingvulcanized polymer are approaching the same values. This is, of course,in contrast to the significant difference exhibited with compositionscontaining only 3 wt% polymer in the polymer/asphalt blend as reflectedby the toughness and tenacity value obtained in Example 1 (comparecompositions 1 and 5B). It follows from this that maximum improvement isrealized when the asymmetric, radial polymer is used in compositions atconcentrations more commonly used in road paving compositions.

While the present invention has been described and illustrated byreference to particular embodiments thereof, it will be appreciated bythose of ordinary skill in the art that the same lends itself tovariations not necessarily illustrated herein. For this reason, then,reference should be made solely to the appended claims for purposes ofdetermining the true scope of the present invention.

Having this described and illustrated the present invention, what isclaimed is:
 1. A bitumen composition comprising an asymmetric, radialpolymer comprising a plurality of arms which contain at least onepolymeric block of a monoalkenyl aromatic hydrocarbon and at least onepolymeric block of a conjugated diolefin and a plurality of arms whichare a conjugated diolefin polymer, said asymmetric, radial polymerhaving a weight average molecular weight of at least 600,000.
 2. Thebitumen composition of claim 1 wherein said asymmetric, radial polymercomprises from about 2 to about 25 block copolymer arms and from about 2to about 25 conjugated diolefin polymer arms, the ratio of said armsbeing within the range from about 5:1 to about 1:5.
 3. The bitumencomposition of claim 2 wherein said asymmetric, radial polymer ispresent in said bitumen composition at a concentration within the rangefrom about 1 to about 12 parts by weight, per 100 parts by weight ofbitumen.
 4. The bitumen composition of claim 3 wherein said monoalkenylaromatic hydrocarbon is styrene.
 5. The bitumen composition of claim 4wherein said polymeric block of a conjugated diolefin is a block ofpolybutadiene.
 6. The bitumen composition of claim 4 wherein saidpolymeric block of a conjugated diolefin is a block of polyisoprene. 7.The bitumen composition of claim 4 wherein said conjugated diolefinpolymer is a homopolymer of butadiene.
 8. The bitumen composition ofclaim 4 wherein said conjugated diolefin polymer is a homopolymer ofisoprene.
 9. The bitumen composition of claim 2 wherein the monoalkenylaromatic hydrocarbon polymer block has a weight average molecular weightwithin the range from about 2,000 to about 50,000 and the conjugateddiolefin polymer blocks have a weight average molecular weight withinthe range from about 5,000 to about 150,000.
 10. The bitumen compositionof claim 9 wherein said conjugated diolefin polymer has a weight averagemolecular weight within the range from about 5,000 to about 150,000. 11.The bitumen composition of claim 2 wherein said asymmetric, radialpolymer is sulfur vulcanized.
 12. The bitumen composition of claim 1wherein said asymmetric, radial polymer comprises from about 8 to about20 arms and the ratio of block copolymer arms to conjugated diolefinpolymer arms is within the range from about 3:1 to about 1:3.
 13. Thebitumen composition of claim 1 wherein said asymmetric, radial polymerhas a weight average molecular weight within the range from about600,000 to about 2,500,000.
 14. An asphalt concrete compositioncomprising from about 0.04 to about 1.7 wt% of an asymmetric, radialpolymer, comprising a plurality of arms which contain at least onepolymeric block of a monoalkenyl aromatic hydrocarbon and at least onepolymeric block of a conjugated diolefin and a plurality of arms whichare a conjugated diolefin polymer, having a weight average molecularweight of at least 600,000, from about 3.3 to about 9.96 wt% bitumen andfrom about 90 to about 96 wt% bitumen and from about 90 to about 96 wt%aggregate.
 15. The asphalt concrete composition of claim 14 wherein saidasymmetric, radial polymer comprises from about 2 to about 25 blockcopolymer arms and from about 2 to about 25 conjugated diolefin polymerarms, the ratio of said arms being within the range from about 5:1 toabout 1:5.
 16. The asphalt concrete composition of claim 15 wherein saidasymmetric, radial polymer has from about 8 to about 20 arms and theratio of block copolymer arms to conjugated diolefin polymer arms iswithin the range from about 3:1 to about 1:3.
 17. The asphalt concretecomposition of claim 16 wherein the monoalkenyl aromatic hydrocarbon isstyrene and the conjugated diolefin is isoprene.
 18. The asphaltconcrete composition of claim 17 wherein said asymmetric, radial polymerhas a weight average molecular weight within the range from about600,000 to about 2,500,000.
 19. The asphalt concrete composition ofclaim 16 wherein the monoalkenyl aromatic hydrocarbon is styrene and theconjugated diolefin is butadiene.
 20. The asphalt concrete compositionof claim 19 wherein said asymmetric, radial polymer has a weight averagemolecular weight within the range from about 600,000 to about 2,500,000.21. The bitumen composition of claim 12 wherein said asymmetric, radialpolymer is present in said bitumen composition at a concentration withinthe range from about 3 to about 6 parts by weight, per 100 parts byweight of bitumen.
 22. The bitumen composition of claim 13 wherein saidasymmetric, radial polymer has a weight average molecular weight withinthe range from about 750,000 to about 1,800,000.
 23. The asphaltconcrete composition of claim 20 wherein said asymmetric, radial polymerhas a weight average molecular weight within the range from about750,000 to about 1,800,000.